Oxidation mechanism of the ammonium-fluoride-treated Si(100) surface

High-resolution electron energy loss (HREEL) spectroscopy, Auger electron spectroscopy, and low-energy electron diffraction have been employed to examine the initial oxidation stage of ammonium-fluoride-treated Si(100) surfaces exposed to air. The NH4F treatment results in a hydrogen-terminated surface, as shown by the presence of the Si–H stretch (2100 cm−1), SiH2 scissor (910 cm−1), and SiH bend (650 cm−1) in the HREEL spectra. Initial oxidation on this surface occurs through oxygen insertion in the silicon backbonds, as indicated by the presence of the asymmetric bridge-bonded oxygen stretch in the region between 1060 and 1160 cm−1. Oxygen is observed in both surface and bulk bridge-bonding sites for even the shortest air exposures, suggesting that initial oxygen uptake is not taking place in a layer-by-layer fashion. Auger electron spectroscopy shows a slow uptake of oxygen over the first few days of air exposure, followed by more rapid oxidation. Over the first two weeks of air exposure, the Si–H stretch at 2100 cm−1 gradually disappears, accompanied by the appearance of oxygen backbonded Si–H peaks around 2250 cm−1. This behavior indicates that, despite the increase in oxygen uptake, the hydrogen termination remains intact, and further confirms that oxygen uptake occurs through insertion in the silicon backbonds. Water is found to play a significant role in the initial oxidation.